AC motors are used in a variety of applications, including vehicle applications such as traction control. The AC motors used in vehicle applications are typically controlled (e.g., via a voltage source inverter) such that the motor phase currents are sinusoidal. These AC motors are generally permanent magnet motors designed to have a sinusoidally-shaped back electromagnetic field (emf) waveform. Supplying a sinusoidally shaped input current to the AC motor typically produces the highest average torque without additional low-frequency harmonies which can be a source of torque pulsations in the AC motors.
When operating the AC motor at higher frequencies over a fundamental output cycle of the AC motor, thermal stresses in the voltage source inverter are usually evenly distributed among the switch network (e.g, among the six switches based on a transistor-diode combination). At lower frequencies, including stall, the transient thermal impedance of these switches may occur over periods that are less than the period of the fundamental output cycle. As a result, the thermal stresses in the voltage source inverter may be unevenly distributed for short time periods, and some switches respond to the uneven distribution with elevated temperatures that limit the current carrying capacity of such switches. Depending on the operating range of the fundamental output frequencies, the output current capacity of the voltage source inverter at low frequencies, including direct current (DC), may be substantially reduced in comparison with the output current capacity of the voltage source inverter operating at higher fundamental frequencies.
Accordingly, it is desirable to provide a method for controlling AC motor torque output at lower operating frequencies. Additionally, it is desirable to provide a voltage source inverter that controls an AC motor to increase torque output at lower operating frequencies. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.